Ripples on a dune in Eureka Valley, California (photograph by Terrence Moore).

A worldwide inventory of deserts has been developed using images from the Landsat
satellites and from space and aerial photography. It defines five basic types of
dunes: crescentic, linear, star, dome, and parabolic.

The most common dune form on Earth and on Mars is the crescentic. Crescent-shaped
mounds generally are wider than long. The slipface is on the dune's concave side.
These dunes form under winds that blow from one direction, and they also are
known as barchans, or transverse dunes. Some types of crescentic dunes move
faster over desert surfaces than any other type of dune. A group of dunes moved
more than 100 meters per year between 1954 and 1959 in China's Ningxia Province; similar rates
have been recorded in the Western Desert of Egypt. The largest crescentic dunes on Earth, with
mean crest-to-crest widths of more than 3 kilometers, are in China's Taklimakan Desert.

These crescentic dunes of coastal Peru are migrating toward the left (photograph
by John McCauley).

Straight or slightly sinuous sand ridges typically much longer than they are wide
are known as linear dunes. They may be more than 160 kilometers long. Linear
dunes may occur as isolated ridges, but they generally form sets of parallel
ridges separated by miles of sand, gravel, or rocky interdune corridors. Some
linear dunes merge to form Y-shaped compound dunes. Many form in bidirectional
wind regimes. The long axes of these dunes extend in the resultant direction of
sand movement.

Linear dunes advance on small playas east of Lake Eyre in the Simpson Desert of central
Australia (photograph by C. Twidale).

Linear dunes in the western deserts of Egypt (photograph by Carol Breed).

Radially symmetrical, star dunes are pyramidal sand mounds with slipfaces on
three or more arms that radiate from the high center of the mound. They tend to
accumulate in areas with multidirectional wind regimes. Star dunes grow upward
rather than laterally. They dominate the Grand Erg Oriental of the Sahara. In
other deserts, they occur around the margins of the sand seas, particularly near
topographic barriers. In the southeast Badain Jaran Desert of China, the star
dunes are up to 500 meters tall and may be the tallest dunes on Earth.

Star dunes, such as these of the Namib, indicate the winds that formed them blew
from many directions (photograph by Georg Gerster).

Oval or circular mounds that generally lack a slipface, dome dunes are rare and
occur at the far upwind margins of sand seas.

U-shaped mounds of sand with convex noses trailed by elongated arms are parabolic
dunes. Sometimes these dunes are called U-shaped, blowout, or hairpin dunes, and
they are well known in coastal deserts. Unlike crescentic dunes, their crests
point upwind. The elongated arms of parabolic dunes follow rather than lead
because they have been fixed by vegetation, while the bulk of the sand in the
dune migrates forward. The longest known parabolic dune has a trailing arm 12
kilometers long.

Small crescentic dunes occur on the crests of these complex dome dunes of Saudi
Arabia's Empty Quarter (photograph by Elwood Friesen).

Ripples and horns of this crescentic dune in Egypt indicate that the dune is
moving right to left (photograph by John Olsen).

Occurring wherever winds periodically reverse direction, reversing dunes are
varieties of any of the above types. These dunes typically have major and minor
slipfaces oriented in opposite directions.

All these dune types may occur in three forms: simple, compound, and complex.
Simple dunes are basic forms with a minimum number of slipfaces that define the
geometric type. Compound dunes are large dunes on which smaller dunes of similar
type and slipface orientation are superimposed, and complex dunes are
combinations of two or more dune types. A crescentic dune with a star dune
superimposed on its crest is the most common complex dune. Simple dunes represent
a wind regime that has not changed in intensity or direction since the formation
of the dune, while compound and complex dunes suggest that the intensity and
direction of the wind has changed.